Just to save you time - I'd already checked Great Milton baptisms for Robert, without any luck.
I've just seen your latest post. I've found Robert & Diana's marriage in Gt Haseley in 1827 and found the baptisms for their first children - twins called William & John. I'd say that the fact that one of them is called William and the fact that twins run in the family would point to the baptism in Stoke Talmage to William & Alice being the right one. I'd like to bet that John is probably the name of Diana's father!
The other children baptised to William & Alice in Stoke Talmage are:
June 27 1784 John
Oct 9 1785 Nanny
Feb 27 1787 Thomas
Mar 22 1789 Alice
Aug 1 1790 Martha
Aug 25 1794 Joseph & James, twin sons
Oct 12 1795 James
Dec 11 1796 William
July 4 1798 Robert
Aug 14 1799 Hannah
Oct 31 1800 Phoebe
Apr 20 1802 Lucy
May 1 1803 Isaac
Nov 10 1805 Susanna
Jan 6 1809 Robert & Richard, twin sons (first mention of 'Pyrton parish')
Phew! There are so many children, I was beginning to think that there might be 2 William & Alice Smiths having children at the same time, but they all seem to fit in timewise!!
There are no burials to be found in Stoke Talmage for the first James & Robert - perhaps they were buried in Pyrton.Neither is there any sign of William & Alice's marriage there.